1. Field of the Invention
The present invention relates to a scanning electron microscope for scanning a specimen placed on a specimen stage by an electron beam and obtaining image information about the specimen based on the scanning.
2. Description of Related Art
In recent years, scanning electron microscopes have been used not only in scientific purposes but also for product inspections during various process steps in a manufacturing plant or at the time of shipment. For example, in semiconductor fabrication plants, scanning electron microscopes are used to inspect photolithographically defined silicon wafers. Inspections making use of high magnifications and high resolutions of scanning electron microscopes have become increasingly indispensable.
On the other hand, environments where a scanning electron microscope within a manufacturing plant is installed are different from environments where a scanning electron microscope is installed for scientific purposes. In the former environments, vibrations from the floor surface inevitably enter the instrument. This is undesirable for high-magnification, high-resolution scanning electron microscopes. Furthermore, strengthening of the floor surface and improvement of the installation environments of scanning electron microscopes may involve difficulties because of costs or because the purpose is to inspect finished products incorporated in a production process.
To reduce the effects of vibrations, the rigidity of the scanning electron microscope (especially, the rigidity of the portion located between the specimen stage on which a specimen is placed and the specimen chamber incorporating the stage) has been improved. The specimen is irradiated with an electron beam through the specimen chamber. Thus, variations in the relative position due to vibrations of the electron beam hitting the specimen have been decreased.
In another technique disclosed, for example, in JP 2003-051435 for reducing the effects of vibrations on the image information, vibrations occurring between the specimen stage and specimen chamber are detected. Based on the resulting detection signal, the beam hit position is corrected. In this way, the effects of relative positional variations of the portion located between the specimen stage and the specimen chamber on the image information are reduced.
In a further technique disclosed, for example, in U.S. Pat. No. 6,043,490 for reducing the effects of vibrations on image information, vibrations occurring between the specimen stage and specimen chamber are detected. Based on the resulting detection signal, the obtained image information is processed to reduce the effects of relative positional variations of the portion located between the specimen stage and the specimen chamber on the image information.
However, the effects of variations in the relative position between the specimen stage and the specimen chamber on the image information are not easily removed with any one of the prior art techniques described above. In particular, the scanning electron microscope has a resolution on the order of nanometers. Even if the rigidity of the portion located between the specimen stage and the specimen chamber is improved, it is difficult to remove vibrations comparable to such a resolution. Furthermore, when vibrations of the portion located between the specimen stage and the specimen chamber are detected and the beam hit position is controlled using the resulting detection signal, error is produced due to delay of the control. In addition, in order to detect vibrations occurring between the specimen stage and the specimen chamber and to remove the effects of vibrations on the obtained image information using the detection signal by an image processing technique, it is necessary to perform complex image processing.
It can be seen from the foregoing that it is important how to achieve a scanning electron microscope capable of detecting variations in the relative position between a container having a specimen therein and a specimen stage which is located inside the container and on which the specimen is placed, the microscope being further characterized in that the effects of vibrations on image information can be removed easily and reliably.